DC or AC synchronous motors using DC auxiliary exciting magnetic poles are widely applied in driving machines or industrial equipment as well as in driving various electrical carriers or public transportation carriers due to their low cost, good operating performance, high efficiency, and ease of control. Conventionally, switching between multiple series and parallel combined motors is highly reliable with good effect, as is usually seen in the speed control of railed electrical cars. However, such conventional speed controls can only provide multi-stage changes in series, parallel or mixed combinations, but not continuous linear control. Although the intermediate discontinuous sections can be adjusted by variable resistors, the resistors have high heat loss as well as a negative effect on the electromechanical characteristics. Therefore, a need exists for a circuit which possesses a good continuous adjustment between the stages of series, parallel, or mixed series and parallel combinations, as well as economic efficiency and a good structure, using solid state active control components to eliminate the disadvantages of short life and high cost of the conventional series and parallel switching electromechanical components.